This invention relates to a direct injection internal combustion engine which has at least two intake valves per cylinder mounted side by side, a plurality of exhaust valves, a more or less coaxial spark plug, an injection valve, and a piston. The invention also relates to a process for mixture preparation in a direct injection internal combustion engine.
An internal combustion engine and a process such as this are known, for example, from The Ford PROCO Engine Update,xe2x80x9d Scussel A. J. et al., SAE 780699. In this instance fuel is injected directly into a cylinder of an internal combustion engine by means of a high-pressure injection nozzle. However, the time required for preparation of a homogeneous lean mixture limits the injection time. On the other hand, in order to obtain a local area with a combustible fuel/air mixture, it is necessary to be only very late in introducing the fuel, something which creates problems in mixture formation.
In this context it is the object of this invention to design an improved direct injection internal combustion engine and a process of mixture preparation characterized in homogeneous lean fuel operation and in stratified lean fuel operation by low fuel consumption and by low exhaust emissions.
To attain this end the injection valve is mounted between two adjacent intake valves and the section of the cylinder wall adjoining these intake valves and the piston has an oblong combustion chamber trough, which extends more or less in the direction of the vertical projection of the injection stream emerging from the injection valve onto the face of the piston and has a step extending more or less crosswise. This step divides the combustion chamber trough into a first area remote from the injection valve guiding the flow of fresh air and an area near the injection valve guiding the injection stream, the first and the second areas being situated on different levels. Division into two areas results in especially efficient preparation of the mixture in the combustion chamber and in better combustion accompanied by low pollutant values. For this purpose the fresh air tumble flowing into the combustion chamber, which enters through the intake openings controlled by the adjacent intake valves and flows downward on the cylinder wall opposite intake openings toward the piston, and the fuel, which is injected through the injection valve early or late, depending on the operating condition of the internal combustion engine, is homogeneously mixed or, again, is prepared as a stratified charge, moves to the spark plug and is ignited there. In the case of homogeneous lean fuel operation the oblong combustion chamber trough with transverse step causes the fresh air tumble flowing into the combustion chamber to move through the first combustion chamber area far from the injection valve and to be deflected upward by the step so that the fuel injected into the combustion chamber opposite the tumble flow may be injected into the combustion chamber trough. And in stratified lean fuel operation the fuel injected by the injection valve in advance of the tumble flow may be injected into the second area of the combustion chamber upstream from the step near the injection valve. As soon as fuel particles reach the step, they are mixed thoroughly with the air flowing in the opposite direction, so that a homogeneous fuel mixture spray is generated near the step, while a distinct lean mixture is present in the rest of the combustion chamber. Subsequent compression movement of the piston then moves the fuel mixture spray to the spark plug, where it is ignited.
In accordance with an advantageous development of the invention, the intake valves should be spaced the maximum distance from each other to sustain the tumble flow of fresh air. This feature also favors mounting of the injection valve between the intake valves.
The injection valve should be mounted at an angle of approximately 30 to 80 degrees to the axis of the cylinder, so that the oblong combustion chamber trough up to the step can be reached by injected fuel. In addition, the injection valve may be bent at an angle causing the injection stream to be deflected in the direction desired so that the central axis of the injection stream is positioned at an angle of approximately 45 to 70 degrees to the axis of the cylinder. As a result of the inclination and/or bend of the injection valve the injection stream may be adjusted so that it is directed into the tumble flow at a first crankshaft angle for homogeneous lean fuel operation and in advance of the tumble flow in a second crankshaft angle area, so that optimum mixture preparation is achieved for both stages of operation.
The injection valve injects fuel into the cylinder for the sake of homogeneous lean fuel operation during the intake stroke at a crankshaft angle of approximately 440 to 280 degrees, depending on engine speed, before the piston reaches upper dead center, since in this way the fuel is drawn approximately into the center of the fresh air flow prevailing in the combustion chamber so that the fuel and the fresh air may be homogeneously mixed together.
And for stratified lean fuel operation the injection valve injects fuel into the cylinder during the compression stroke with the crankshaft at an angle of approximately 120 to 30 degrees, before the piston reaches upper dead center. Hence the fuel is injected for the most part in advance of the fresh air tumble flow, is stabilized by the fresh air flow within the area of the combustion chamber trough as an ignitable fuel charge spray, and lastly is transported to the spark plug. In the process the oblong combustion chamber trough with step extending crosswise serves to guide or concentrate the fresh air tumble flow and to stabilize the ignitable mixture spray formed from the flow. This process can be completed despite the relatively late injection time.
By preference the step which divides the combustion chamber trough into a first area remote from the injection valve and a second area near the injection valve takes place more or less below the spark plug. As a result the first area allows efficient guidance of the fresh air tumble flow and the centrally positioned second step favorable deflection of the fresh air flow, while the second area effects adequate stabilization of the fuel charge spray near the spark plug. By special preference the first area of the combustion chamber trough is designed to be deeper than the second area, since this feature facilitates deflection of the fresh air tumble flow by the step representing transition from the deep area to the shallow area. However, the first area of the combustion chamber trough may, of course, also be designed to be shallower than the second area. Lastly, each of the two areas of the combustion chamber trough may also be designed to incline at a slight angle toward the edge of the piston. This results in particularly gentle flow of fresh air into the first area and of fuel into the second area of the combustion chamber trough.
In accordance with a development of the invention the oblong combustion chamber trough widens in the direction of the injection stream. As a result, the fresh air tumble flow is concentrated or focused in the combustion chamber trough, and this facilitates thorough mixing of fresh air and fuel. Similarly, the first area of the combustion chamber trough may be wide, more or less parabolic, in shape, while the second area may be in the form of a narrow corridor. These shapes are optimally adapted to the fresh air flowing in through at least two intake valves and correspondingly to the fuel injected through the injection valve.
Additional optimization of mixture preparation in this direct injection internal combustion engine is achieved by forming on the step extending more or less crosswise in the oblong combustion chamber trough a fin which extends over the entire length of the step. The fresh air tumble flow is especially favorably guided and maintained for a prolonged period during compression and injection, and the fuel stream, which is injected more or less in a straight line, is deflected upward by the fin. These two factors result in very decidedly improved mixture preparation, so that a direct injection internal combustion engine characterized by markedly low fuel consumption and especially low exhaust gas emissions is created.
Further improvement is achieved if the fin is, for example, in the form of a sawtooth in cross-section, with the tip of the sawtooth fin pointing in the combustion chamber toward the intake valve or injection valve side. The fresh air in tumble flow is thus guided with highly favorable results over the curved back of the fin during compression and injection and as a result the flow is sustained for a prolonged period. In addition, the fuel stream injected more or less in a straight line is deflected distinctly upward by the tip of the fin. If the curved back of the sawtooth fin facing the exhaust valve side has a radius corresponding to approximately one-half the height of the fin relative to the second area of the combustion chamber trough, in the internal combustion engine claimed for the invention especially efficient coordination is achieved of the fresh air in tumble flow in the first area of the internal combustion engine with the fuel stream injected more or less in a straight line into the second area of the combustion chamber trough and deflected upward, since the fresh air in tumble flow distinctly supports transport of the fuel spray formed above the first area of the combustion chamber trough or above the fin toward the spark plug.
If the step with the fin is mounted at a distance of approximately {fraction (1/9)} to ⅕ of the piston diameter from the more or less coaxial spark plug in vertical projection onto the face of the piston, the tip of the spark plug in the upper dead center of the piston behind the fin may be dipped into the first area of the combustion chamber trough.
Preferably the transition from the step with fin to the first area of the combustion chamber trough extends at an angle of approximately 2 to 30 degrees to the axis of the cylinder so as to ensure optimum deflection of the tumble flow at this point. And by special preference the surface of the step with fin is slightly concave on the side facing the first area of the combustion chamber trough, since the concavity of the step effects even more pronounced stabilization of the fresh air tumble flow in the first area of the combustion chamber trough. The suitably rounded or chamfered fin may be encompassed by the outline of the piston but may also project above the surface outline of the piston.
The first area of the combustion chamber trough near the step with fin is preferably wider than the fin. And by special preference the first area of the oblong combustion chamber trough near the plurality of exhaust valves is designed so that the disks of the exhaust valves in the upper dead center of the piston may be introduced into this area. This makes certain that the disks of the exhaust valves are not seated on the piston in the event of change in the valve control times.
It is expedient for the outlet opening of the injection stream to be positioned in the projection onto the face of the piston in advance of the fin and as far away as possible from the axis of the cylinder.
The object of the invention is also attained by a process of mixture preparation in a direct injection internal combustion engine as specified in claim 14.
In this process fresh air forms a tumble flow which enters the first area of the combustion chamber trough near the edge of the piston, is accelerated by the first area narrowing toward the center of the piston in the direction of flow, and finally is deflected in the direction of the spark plug by the step of the combustion chamber trough.
In addition, in stratified lean-fuel operation the tumble flow streams into the first area of the combustion chamber and during the compression stroke and injection is kept by the step above the first area of the combustion chamber trough.
In a development of the process, in stratified lean-fuel operation fuel from the injection valve is first injected in a straight line in the direction of the second area of the combustion chamber trough into the fresh air streaming as tumble flow through the first area of the combustion chamber trough and then is deflected by the step with fin in the direction of the spark plug, there being formed above the second area of the combustion chamber trough or above the fin in sawtooth form a fuel spray which, sustained by the fresh air tumble flow, is transported in the direction of the spark plug and is mixed with fresh air.
Lastly, the process claimed for the invention is to be carried out so that toward the end of the compression stroke a flow velocity of approximately 3 to 6 meters per second is maintained on the spark plug, so that favorable conditions for ignition of the fuel spray prevail in the combustion chamber.